1. Field of the Invention
The present invention relates to liquid crystalline compounds and mixtures as well as electro-optical devices.
2. Description of the Art
Liquid crystals are used primarily as dielectrics in indicating devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well-known to the person skilled in the art and can be based on various effects such as, for example, dynamic scattering, the deformation of aligned phases (DAP cells), the SchadtHelfrich effect (TN cells, i.e, twisted-nematic, and STN cells i.e, super twisted-nematic), the guest/host effect (guest/host cells), a cholesteric-nematic phase transition (phase-change cells) or the SBE effect (super birefringence effect).
Cholesteric liquid crystals are used, for example, in the above-mentioned phase-change cells. Further, cholesteric additives or other chiral substances with suitable choice of the concentration can also be used to improve the electro-optical properties of liquid crystals for TN cell indicators. Mematic liquid crystal components can be used in all of the aforementioned applications.
The aforementioned indicating devices generally have response times in the order of several milliseconds or more. In order to improve the response times of indicating devices, liquid crystals with ferroelectric properties have recently also been used. In this application there are used chiral smectic phases, for example, smectic C, F or I phases, mainly smectic C phases. Hitherto, however, relatively few of such liquid crystals have become known and their stability is often inadequate.
In order to be suitable for use in indicating devices liquid crystals should have a good chemical and thermal stability and a good stability towards electrical fields and electromagnetic radiation. Further, they should be colorless and have low viscosities and should give short response times and a high contrast in indicating devices. Furthermore, the liquid crystals should have a suitable mesophase, for example, a cholesteric or a suitable chiral smectic phase, at the usual operating temperatures. Because liquid crystals are usually employed in mixtures, it is important that the components have a good miscibility with one another. Other properties such as, for example, the threshold potential, the dielectric anisotropy and the optical anisotropy, must fulfill different conditions depending on the type of cell used. Liquid crystals with ferroelectric properties preferably have a negative dielectric anisotropy or a small absolute value of the dielectric anisotropies.